This weak reactivity was unlikely to be a consequence of the relatively low amino acid sequence identity (76%) of S1 between MHV-JHM and MHV-S, because the level of antibody to S1 in anti-MHV-A59 serum was equivalent to or higher than that in anti-MHV-S serum despite the even lower S1 identity (73%) between MHV-A59 and MHV-S.